The invention relates to the alignment of an object having a feature sufficient to distinguish various angular orientations of the object, e.g., a circular semiconductor wafer having a flat on its edge.
Semiconductor wafer production requires, as one of its steps, the alignment of the wafers. Alignment of the flat on the edge of a wafer has often been accomplished by using a roller that rotates a wafer as long as it contacts the wafer's circular surface and ceases rotating the wafer once it contacts the wafer's flat surface, e.g., as shown in Foulke et al. U.S. Pat. No. 4,493,606, which is hereby incorporated by reference.
Judell et al. U.S. Pat. No. 4,457,664 discloses a different method of aligning wafers. The wafer is rotated so that its edge passes a capacitive sensor to obtain information on the distance of the edge from the axis of rotation; based on that distance versus angular orientation information, the wafer is realigned with respect to its centroid. The wafer is then rotated a second time to obtain further information relating to the present angular position of a flat on the edge of the wafer, and a threshold value is used to determine the location of the midpoint of the flat.